Previous studies by our group and other investigators have shown that induction of delayed type hypersensitivity reactions to chemical haptens or microbial antigens at the site of neoplasms involving the skin of man or guinea pigs results in tumor regression. It has been proposed that effectors in the tumor regression are non-specifically activated leukocytes in the inflammatory infiltrate of the delayed hypersensitivity reactions. Support for this view is the observation of a selective cytotoxicity of non-specifically activated large mononuclear cells (monocytes, macrophages) of rats and mice against neoplastic cells. Tha aim of the proposed studies is to elucidate further the interaction of non-specifically activated leukocytes (monocytes, lymphocytes and granulocytes) with neoplastic and normal cells. Previous studies on the effect of serum of tumor-bearing animals on tumor cell leukocyte interaction will be continued and extended to man. The work will be carried out in an in vitro tissue culture system. The proposed studies have bearing upon central questions pertaining to host defense mechanisms against neoplastic cells. As such, they have important practical as well as theoretical implications.